|Plan of Pen Llystyn Fort, oriented with north at the top. Judging from the number of barrack blocks which are seen clearly on the plan, each with ten double-rooms and a centurial block at the end nearest the defences, the fort housed a miliary infantry cohort with a nominal 960 foot-soldiers. (adapted from Webster The Roman Imperial Army fig.48).|
Probable road: N (12) to SEGONTIVM|
Probable road: ESE (14) to Tomen y Mur
The fort stands on a west facing slope on the east bank of the Afon Dwyfach, just north of Bryncir on the A487 trunk road from Porthmadog to Caernarfon. It lies within the angle formed where the postulated Roman military road between the forts at Segontium (Caernarfon) and Tomen-y-Mur makes a marked change of alignment. The road alters heading from south-south-east to east-south-east, about a kilometer to the south of the site. The fort faces south-west across the river valley and was obviously sited to police the Ganganorum Promontorium (Lleyn Penninsula), possibly the home of the Gangani tribe.
Unfortunately, the site is now almost completely lost due to gravel-working.
There is a temporary camp on the opposite bank of the Afon Dwyfach at Derwydd-bach Farm (SH4745), just to the north-east, and a substantial Roman building at Tremadog (SH5540), along the line of the postulated road east to Tomen-y-Mur.